This study will continue to explore the interrelationships between malignancy and immunity in the Gross passage A virus-mouse system. Attempts will be made to determine what specific factors are involved in the leukemogenic process. This study will incorporate a variety of experimental approaches: 1) The virus-host interactions will be explored by analyzing the virus, its sources, routes of introduction to the host, its sites of action, and dissemination throughout the host; 2) The immune response mechanism to virus during leukemogenesis will be studied; and 3) Attempts will also be made to heighten the immune response to bring about a remission or a rejection of the malignancy. BIBLIOGRAPHIC REFERENCES: Mariani, T.: Skin as a vehicle for horizontal transmission of Gross passage A leukemia in adult mice. A unique microenvironment? In: Fifth International Conference on Lymphatic Tissue and Germinal Centers in Immune Reactions, held in Tiberias, Israel, June 24-30, 1975. M. Feldman and A. Globerson (Eds.) Plenum Press, New York (In Press). Mariani, T.: Effect of Gross passage A preleukemic thymic implants on reconstitution of thymectomized C3h/Bi mice. Fed. Proc. 34:869, 1975.